The present invention relates generally to air core gauges, and more specifically to techniques for determining the position of a magnetic rotor in such an apparatus including at least three coils in proximity to the rotor.
Typical analog displays, such as those used in vehicle instrument panels, utilize air core gauges or stepper motors to position pointers in relation to sensor values. Conventional air core gauge mechanisms typically include a rotor formed of a substantially circular disk of magnetized material that is fixed to a spindle, wherein the rotor is surrounded by at least two coils of wire, with at least one coil typically perpendicular to another of the coils. When electric current passes through the coils, a magnetic field is produced that exerts a force on the rotor. The angular direction of the magnetic field produced by the coils primarily depends on the number of ampere-turns in each of the coils, wherein the resultant magnetic field can be represented by the vector addition of the fields produced by each of the coils.
Stepper motors are inherently more accurate than air core gauges, although to achieve the higher accuracy, stepper motors typically incorporate a high stepdown gear ratio between the magnetic rotor and the pointer shaft or multi-pole rotors in combination with a geartrain. The additional parts required in typical stepper motors as compared with air core gauges undesirably increases the mechanism cost and often times necessitates, at least from a cost standpoint, the use of air core gauges.
A conventional two-coil air core gauge is typically driven by one of two known techniques. According to a first known technique, as shown in FIG. 1, the two coils are designated by reference numerals 36 and 38. Coil 38 is biased to a fixed voltage VIGN through resistor 34. The resistance of resistor 32 typically varies in relation to a physical parameter such as fuel level. Resistor 30 supplies current to coil 36 from VIGN. The voltage across coil 36 is determined by a voltage divider comprised of resistor 30 and the parallel combination of resistor 32 and the resistance of coil 36, and the current flowing through coil 36 varies in proportion to the voltage thereacross. Coil 36 and coil 38 are arranged to generate orthogonal magnetic vectors that sum to form a resultant magnetic vector. As the current flowing through coil 36 varies in response to the changing resistance of resistor 32, the vector component of the magnetic field generated by coil 36 similarly varies. The direction and magnitude of the magnetic field resulting from vector addition of the field components generated by coils 36 and 38 thus varies in relation to the changing resistance of resistor 32. The magnetic rotor aligns with the resultant magnetic field direction, and its rotational position is thus determined by the direction of the resultant magnetic field which is determined by resistor 32.
According to a second known technique for driving a two-coil air gauge, as shown in FIG. 2, a signal on line 50 from a sensor (not shown), typically a signal with a frequency varying with a vehicle parameter, is converted to a corresponding analog voltage through a frequency-to-analog converting circuit 52. The resultant analog signal is provided as an input to a sine/cosine drive circuit 56, whereby the sine/cosine drive circuit 56 generates a current flowing through signal path 58 proportional to the cosine of the desired angle of deflection of the rotor, and a current flowing through signal path 60 proportional to the sine of the desired angle of deflection of the rotor. Coils 67 and 64, in response to the currents flowing through signal paths 58 and 60 respectively, develop magnetic fields with sine and cosine component magnetic vectors correlating to the desired pointer rotation. Various other techniques which are not set forth here are also known and are used to drive air core gauges.
Air core gauge error sources include hysteresis, pointer staking errors and linearity errors. Pointer staking and linearity errors can be minimized with a calibration process, although calibration of the mechanism typically adds investment and cycle time to the system cost. Hysteresis errors, on the other hand, typically cannot be compensated for in an open-loop system, wherein most conventional drive techniques for air core gauges, including those set forth above, are typically open-loop systems in which actuation currents are applied to the coils without the use of any feedback information as to the actual pointer position to allow for corrections to the values of the currents. If the center of mass of the pointer does not lie on the axis of the pointer spindle, the weight of the pointer will generally cause the pointer to sag from the angular position in which the magnetic field of the rotor aligns with the resultant magnetic field of the two coils.
One approach to addressing such hysteresis and other errors to thereby improve pointer position accuracy is disclosed in U.S. Pat. No. 5,489,842 (hereinafter ""842 patent), owned by the assignee of the present invention, and the disclosure of which is incorporated herein by reference. The ""842 patent discloses an air core gauge 411, as illustrated in FIG. 3, which includes a generally circular or cylindrical magnetic rotor 410 driven by two coils 412 and 414 about an axis 409 (shown in phantom), which are wound around perpendicular axes, B-F and O-D, respectively, and mounted within the proximity of the rotor 410. In addition to the normal rotation drive signal (not shown), coil 412 is coupled to a high frequency AC signal source (not shown) such that a high frequency current is superimposed onto the portion of the drive signal applied to coil 412. Since coils 412 and 414 are perfectly perpendicular, there is no magnetic coupling of the AC input signal from coil 412 to 414. However, rotor 410 provides a magnetic flux linkage between coils 412 and 414, thereby inducing a coupled AC output signal on coil 414 in response to the AC input signal on coil 412. Since the rotation drive signal is substantially DC, the rotation drive signal does not cause signal coupling between coils 412 and 414. Thus, because the frequency of the injected AC current is much higher than the frequency content of the nominally DC currents used to drive the rotor 410 to cause torque in the mechanism, the technique disclosed in the ""842 reference makes it possible to simultaneously drive the rotor 410 to a desired position while determining the position of rotor 410 using filters (not shown) to separate the two activities.
The magnetic flux linkage between coil 412 and coil 414 is proportional to sin(i)*sin(j), where i is the angle between the north pole 416 (or south pole 418) and a line drawn through points B and F, and j is the angle between the north pole 416 (or south pole 418) and a line drawn through points D and O. The magnetic flux linkage between coils 412 and 414 is further dependent upon the rotational position of rotor 410, so that the magnitude and phase of the AC output signal in coil 414 is accordingly dependent upon the rotational position of rotor 410. Thus, a measurement of the AC output signal in coil 414, or the ratio between the input and output AC signals, can be used to make a determination of the rotational position of rotor 410, and therefore the position of a pointer or other mechanism attached to the rotor 410.
While various causes of pointer position error, including hysteresis, can be compensated for with a closed-loop system of the type illustrated in FIG. 3, such systems have a number of drawbacks associated therewith. For example, the apparatus 411 illustrated in FIG. 3 and disclosed in the ""842 reference has inherent accuracy limitations. More specifically, the feedback signal (output AC signal) is a substantially sinusoidal signal and, with two orthogonal coils, the resolution of the feedback mechanism is limited by the diminishing incremental change in magnitude of this signal as it approaches peaks and valleys. Moreover, the geometrical shape of the rotor 410 as well as its material composition are not optimal for rotor position resolution. What is therefore needed is an improved apparatus for maximizing angular rotor position resolution in an air core gauge, and for minimizing angular rotor position errors, that does not suffer from the drawbacks of known rotor position determination systems.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, an apparatus for minimizing angular rotor position errors in an air core gauge comprises an air core gauge having at least three coils disposed proximate to a magnetic rotor with at least one of the coils disposed non-orthogonal relative to the remaining coils, wherein the rotor rotates in response to a composite magnetic field resulting from low-frequency current flowing through one or more of the coils. Means for inducing a probe signal separate from the low-frequency current in one of the at least three coils is also provided, wherein the rotor magnetically couples the probe signal to the other of the at least three coils to thereby produce separate composite signals therein, along with means responsive to the separate composite signals for determining an angular position of the rotor and adjusting the low-frequency current flowing through the one or more of the coils to thereby minimize rotor angular position errors.
In accordance with another aspect of the present invention, an apparatus for minimizing angular rotor position errors in an air core gauge comprises an air core gauge having at least three coils each disposed proximate to a magnetic rotor defining an axis of rotation therethrough and defining a non-circular cross-section along a plane normal to the axis of rotation. The rotor rotates about the axis of rotation in response to a composite magnetic field resulting from low-frequency current flowing through one or more of the coils. Means for inducing a probe signal separate from the low-frequency current in one of the at least three coils is also provided, wherein the rotor magnetically couples the probe signal to the other of the at least three coils to thereby produce separate composite signals therein, along with means responsive to the separate composite signals for determining an angular position of the rotor and adjusting the low-frequency current flowing through the one or more of the coils to thereby minimize rotor angular position errors.
In accordance with yet another aspect of the present invention, an apparatus for minimizing angular rotor position errors in an air core gauge comprises an air core gauge having at least three coils disposed proximate to a magnetic rotor formed of a combination of high and low permeability magnetic materials, wherein the rotor rotates in response to a composite magnetic field resulting from low-frequency current flowing through one or more of the coils. Means for inducing a probe signal separate from the low-frequency current in one of the at least three coils is also provided, wherein the rotor magnetically couples the probe signal to the other of the at least three coils to thereby produce separate composite signals therein, along with means responsive to the separate composite signals for determining an angular position of the rotor and adjusting the low-frequency current flowing through the one or more of the coils to thereby minimize rotor angular position errors.
One object of the present invention is to provide an improved system for minimizing angular rotor position errors in an air core gauge.
Another object of the present invention is to provide such a system by utilizing at least three coils with at least one of the at least three coils disposed non-orthogonal relative to the remaining coils.
Yet another object of the present invention is to provide such a system wherein the rotor defines a non-circular cross-section along a plane normal to its axis of rotation to thereby enable variable coupling between coils due to a change in mass inside each coil as the rotor rotates.
A further object of the present invention is to provide such a system wherein the rotor is formed of a combination of low and high permeability magnetic materials.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiments.